A conventionally known apparatus for obtaining magnetic field strength without using exciting current is a Halbach-type magnetic circuit. This magnetic circuit is disclosed in “Journal of Applied Physics,” Vol. 86, No. 11, Dec. 1, 1999, “Journal of Applied Physics,” Vol. 64, No. 10, Nov. 15, 1988, and Japanese Patents 2,704,352 and 3,115,243. This Halbach-type magnetic circuit comprises pluralities of permanent magnet segments having different magnetization directions arranged to form a synthetic magnetic field oriented in one direction. In the cylindrical arrangement of pluralities of permanent magnet segments SB1 to SB12 shown in FIG. 17, for instance, a permanent magnet segment SB1 is adjacent to the next permanent magnet segment SB2 having a magnetization direction at a certain angle θ to the magnetization direction of the segment SB1, and the subsequent permanent magnet segments are arranged similarly so that their magnetization directions are changed successively, thereby generating a strong, uniform, parallel magnetic field B in the center hole of a magnetic circuit. The conventionally known Halbach-type magnetic circuits have a cylindrical or rectangular magnetic circuit.
Apparatuses comprising conductive coils or superconductive coils are also known as magnetic field sources for apparatuses for generating as strong a magnetic field as about 1 T (Tesla).
A magnetic-field-generating apparatus needing such a strong, uniform magnetic field will be explained, taking a heat treatment step of wafers with magnetic films in a magnetic field for example.
Magnetoresistive (MR) heads, giant magnetoresistive (GMR) heads, magnetic random access memory (MRAM), etc. generally have structures in which pluralities of ferromagnetic films are laminated on a substrate. For instance, the GMR head has a structure comprising a non-magnetic, insulating film between ferromagnetic films. The MRAM has a structure comprising an antiferromagnetic film, a pinned magnetic film, a non-magnetic conductive film and a free magnetic layer in this order from the side of a substrate. The pinned film and the antiferromagnetic film should have magnetization oriented in one direction as a whole.
This orienting step is carried out just after the formation of films on a substrate by sputtering or reactive deposition, and thus it needs a heat treatment in a uniform, parallel magnetic field. An oriented magnetic field of 0.5 T (tesla) or more is usually necessary to be applied, and an oriented magnetic field of more than 1.0 T is necessary depending on the materials of the pinned film or the antiferromagnetic film.
Described in U.S. Pat. No. 6,303,908 is a furnace for heat-treating wafers while applying an oriented magnetic field. A magnetic field-generating means in, this furnace comprises an electromagnetic coil, to which as large electric current as 500-800 A should be supplied to generate a magnetic field of 1.0 T or more. Accordingly, such a magnetic field-generating means uses large electric power, needing safety means and other facilities, and thus a large cost for generating a magnetic field and a large amount of cooling water to remove heat generated by large electric current. Also, because this magnetic field-generating means comprises an iron core and an electromagnetic coil, it weighs 3 to 5 tons to generate a large magnetic field, resulting in restricted installation sites on floors with small strength. Further, because there is an extremely large leaked magnetic flux in the above structure, there should be a large space in addition to a facility space for the sake of safely. In addition, the apparatus should be enclosed by a magnetic body such as iron, Permalloy, etc. to prevent adverse influence on nearby electronic equipments and humans.
A superconductive coil can generate a magnetic field without using large electric power. Though the superconductive coil consumes less exciting current than an electromagnet, liquid nitrogen or helium should always be consumed to keep superconductivity, resulting in a high operation cost. Also, in a system using a superconductive coil, the variation of a magnetic field turns superconductivity to normal conductivity locally, resulting in heat generation in the coil, and if this state were left to stand, the superconductivity of the entire apparatus would be destroyed. Though the superconductive coil can generate as strong a magnetic field as several teslas to several tens of teslas, the range of a strong leaked magnetic field expands in proportion to its magnetic field strength like the electromagnet. Accordingly, it seriously suffers from the problem of a leaked magnetic field like the electromagnet.
JP 11-25424 A proposes a magnetic field-applying apparatus comprising permanent magnets. However, because this apparatus mostly serves to adjust a magnetic field distribution, it generates only a small magnetic field strength, unusable for an orientation apparatus using a strong magnetic field.
Magnetic circuits using electromagnets or superconductive magnets appear to suffer from problems in terms of the generation of a uniform magnetic field. Thus, a Halbach-type magnetic circuit using only permanent magnets is considered promising to solve the above-mentioned problems.
When a wafer having a magnetoresistive film is heat-treated, as large a magnetic field as 1.0 T or more is generally required to stably improve magnetoresistance. Further, the magnetic field need be applied uniformly and in parallel to the magnetization direction of the magnetic film. However, the conventional magnetic-field-generating apparatus using electromagnets, superconductive magnets or permanent magnets cannot generate a strong, uniform, parallel magnetic field.
The inventors have thus considered the use of a Halbach-type magnetic circuit as such a magnetic-field-generating apparatus. However, there is no precedent of using the Halbach-type magnetic circuit for a magnetic-field-generating apparatus for such a heat-treating furnace, needing to investigate the uniformity and parallelness of a magnetic field in a magnetic circuit hole.
It has thus been found that a magnetic field generated by the Halbach-type magnetic circuit is not necessarily uniform. As shown by MS in FIG. 17, for instance, a magnetic flux has disturbed linearity at a tip end in a progressing direction, resulting in non-parallelness, which is called “magnetization bending.” Magnetic field parallelness, which may be called deviating angle or skew angle, is used as an index of the degree of magnetization bending. This indicates an x component vertical to a main magnetic-field-generating direction, namely a deviation (degree) of a magnetic field-applying direction in a cross section (x-y plane) from the main magnetic-field-generating direction (y direction) in a uniform magnetic field region. It is required that this deviation is as small as possible in a region, in which wafers are subjected to an orientation treatment. Specifically, a magnetic field parallelness of ±2° or more adversely affects magnetic film characteristics.
Particularly, this magnetic-field-generating apparatus has a cylindrical center hole, in which magnetic field parallelness drastically decreases as separating from a longitudinal center. The above magnetic field parallelness of within +2° is satisfied only in a region corresponding to a half-length or less of this cylindrical hole. Accordingly, the magnetic-field-generating apparatus should have a cylindrical hole as long as two times or more a treatment region to surely treat sufficient numbers of articles, resulting in larger size and higher production cost.